1. Technical Field
The present invention relates to a physical section of an atomic oscillator. The physical section is a main part of the atomic oscillator. In particular, the present invention relates to a physical section of a highly-accurate and downsized atomic oscillator in which temperatures of a light source and a gas cell are stabilized.
2. Related Art
Atomic oscillators using alkali metals such as rubidium and cesium need to keep alkali metal atoms in a vapor state with buffer gas in a gas cell when the oscillators use energy transition of the atoms. Therefore, the oscillators operate while maintaining the gas cell, in which the atoms are sealed, at a high temperature. An operating principle of the atomic oscillators is broadly classified into a double resonance method utilizing light exciting alkali metal atoms and microwaves (refer to JP-A-10-284772, as a first example), and a method utilizing quantum interference effect (hereinafter, referred to as coherent population trapping: CPT) produced by two kinds of interfering light (refer to U.S. Pat. No. 6,806,784 B2, as a second example). For example, the atomic oscillator using the CPT according to the second example includes a physical section (an optical system) which is a main part of the atomic oscillator and is composed of a semiconductor laser as a light source, a gas cell, and a light detector as a light detection unit. In the gas cell, alkali metal atoms such as a rubidium atom and a cesium atom that are quantum absorbers are sealed. The semiconductor laser produces two kinds of laser light (coupling light and probe light) having different wavelengths from each other to output the laser light to the gas cell. The atomic oscillator detects how much laser light made incident on the gas cell is absorbed by metal atom gas with the light detector so as to detect atomic resonance, and allows a reference signal of a quartz crystal oscillator and the like to synchronize with the atomic resonance at a control system such as a frequency control circuit, obtaining an output. The light detector is positioned at an opposite side of the side, at which the semiconductor laser is positioned, of the gas cell.
Here, when atomic concentration within the gas cell is varied in the atomic oscillator, a degree of absorption of light to the atomic gas is varied, causing an error of detection of the atomic resonance or an impossibility of detection. Therefore, atomic oscillators that are put into practical use include a heating unit for maintaining vapor of atoms within a gas cell at a constant temperature (80° C., for example) and a temperature controlling system controlling the heating unit. However, as a demand of downsizing an electronic apparatus including an atomic oscillator is increased, the atomic oscillator needs to be downsized. Therefore, the heating unit of the gas cell is also required to be downsized and have a function to maintain the gas cell at a constant temperature.
The aging characteristic of the semiconductor laser extremely deteriorates under a high temperature, whereby the life of the semiconductor laser is shortened. This mainly causes short life of the atomic oscillator and unstable laser light irradiation, thus degrading accuracy of the output frequency of the atomic oscillator. Therefore, the temperature of the semiconductor laser needs to be stabilized by suppressing overheat of the semiconductor laser.
In order to meet such need, U.S. Pat. No. 6,320,472 B1 as a third example and JP-A-6-120584 as a fourth example suggest a physical section of an atomic oscillator which controls temperatures of a light source and the like by using a Peltier element using a Peltier effect by which heat is transferred from one surface to the other surface when direct current is applied.
However, in the physical section of the atomic oscillator in the third example, one surface of a Peltier element is closely attached to a light source to enable temperature control of the light source, but the other surface of the Peltier element is positioned where the other surface does not contribute to the temperature control of the physical section of the atomic oscillator. For example, the other surface of the Peltier element and the gas cell are disposed away from each other, so that the Peltier element hardly contributes to the temperature control of the gas cell. Thus the structure of the physical section of the atomic oscillator does not take advantage of the characteristic of the Peltier element for the temperature control.
In the physical section of the atomic oscillator in the fourth example as well, overheat of a light source is prevented by a heat absorption plate disposed on one surface of a Peltier element, but the other surface of the Peltier element is positioned where the other surface hardly contributes to the temperature control of the physical section of the atomic oscillator. Further, the gas cell is irradiated with light of the light source, of which a temperature is controlled by the Peltier element, through an optical fiber, making hard to downsize the atomic oscillator and increasing a cost of the oscillator.